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- MRU3-11D
- Voltage Protection Relay
- USA
- 150 × 100 × 65 mm
- 1.5 kg
- Xiamen, China
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Product Overview
The Woodward MRU3-11D Voltage Protection Relay monitors three-phase AC voltage and detects abnormal conditions that may affect generators, transformers, motors, and industrial distribution networks. In a generator switchgear panel, voltage supervision is particularly important during startup, load changes, grid disturbances, and the transition between operating modes. A voltage deviation that persists beyond the configured limits may indicate a supply problem or an electrical fault requiring a protective response.
The MRU3-11D supports undervoltage, overvoltage, and phase-related voltage protection. Rather than relying on a single fixed threshold, the relay allows protection settings and operating delays to be configured for the application. This helps engineers distinguish sustained voltage abnormalities from short-duration disturbances that may occur during switching or changes in electrical load.
The relay also supports external alarm and trip signaling, allowing its protection decisions to be incorporated into a wider electrical control scheme. In installations that use the documented communication and analog-output options, voltage information can also be transferred to a supervisory controller for monitoring and event analysis.
When selecting the MRU3-11D for a replacement project, engineers should verify the nominal voltage configuration, auxiliary supply, output assignments, and communication requirements. These details determine whether the relay can be integrated into the existing panel without changing the surrounding protection and monitoring circuits.
Technical Specifications
| Parameter | Specification |
|---|---|
| Manufacturer | Woodward |
| Model | MRU3-11D |
| Product Family | MRU3 |
| Product Type | Voltage Protection Relay |
| Monitored System | Three-phase AC |
| Protection Functions | Undervoltage, overvoltage, and phase-related voltage protection |
| Voltage Measurement | Depends on the selected nominal voltage configuration |
| Operating Frequency | 50/60 Hz, subject to configuration |
| Relay Output | 1 SPDT contact, reported rating 5 A at 250 V AC |
| Digital Outputs | 2, according to the documented configuration |
| Analog Output | 4–20 mA voltage signal, where fitted |
| Communication | RS485 / Modbus RTU, where supported by the installed configuration |
| Auxiliary Supply | 24 V DC or 85–265 V AC/DC, according to configuration |
| Mounting | DIN rail or panel mounting, subject to housing configuration |
| Size | 150 × 100 × 65 mm |
| Weight | 1.5 kg |
Product Features
- Three-phase voltage supervision: Monitors voltage conditions in compatible three-phase electrical systems.
- Undervoltage protection: Detects voltage falling below the configured operating threshold.
- Overvoltage protection: Identifies sustained voltage levels above the selected limit.
- Phase-related monitoring: Supports detection of configured phase-voltage abnormalities.
- Adjustable operating delays: Helps coordinate protection with normal switching and transient conditions.
- Alarm and trip signaling: Provides external signaling through the available relay and digital outputs.
- Analog voltage indication: The documented 4–20 mA option allows a selected voltage measurement to be represented as a current signal.
- Serial communication: RS485 and Modbus RTU support monitoring integration where included in the installed version.
- Configurable operating parameters: Allows the protection settings to be matched to the requirements of the electrical installation.
Working Principle
The MRU3-11D receives voltage signals from the monitored electrical system and evaluates them against the configured protection thresholds. Its internal processing determines whether the measured voltage remains within the acceptable range or has crossed a limit requiring an alarm or trip response.
A typical protection arrangement follows this sequence:
Three-Phase Voltage Inputs → MRU3-11D → Alarm / Trip Output → Control Circuit or Circuit Breaker
During normal operation, the measured phase voltages remain within the configured operating limits. If the voltage drops below the undervoltage threshold, rises above the overvoltage threshold, or develops a phase-related abnormality covered by the selected settings, the relay evaluates the condition and its duration.
The operating delay is important in practical installations. For example, a generator may experience a short voltage disturbance during a load change or switching event. A properly coordinated delay can prevent an unnecessary trip caused by a brief deviation while still allowing a sustained fault to trigger the intended protective action.
When the configured protection condition is satisfied, the relay changes the state of its assigned output. That signal can operate an alarm circuit, initiate a circuit-breaker trip, or inform another controller, depending on the wiring and control logic.
Where the analog-output option is installed, a selected voltage measurement can also be transmitted to a compatible analog input. Where supported, serial communication provides another path for monitoring information. These monitoring functions do not replace the relay’s protection settings or the need to test the physical trip circuit.
Role in an Electrical Protection Architecture
The MRU3-11D occupies the voltage-monitoring and protection layer of an electrical control system. It evaluates voltage conditions and supplies alarm or trip signals to the equipment responsible for the subsequent protective action.
Three-Phase Supply → Voltage Measurement → MRU3-11D → Protection Output → Control Circuit / Circuit Breaker
| System Element | Function |
|---|---|
| Generator or AC Supply | Provides the voltage being monitored |
| Voltage Measurement Circuit | Delivers the required phase-voltage signals |
| MRU3-11D | Evaluates voltage conditions against configured thresholds and delays |
| Auxiliary Power Supply | Powers the relay according to the installed configuration |
| Alarm Circuit | Alerts operators to a configured voltage abnormality |
| Trip Circuit | Transfers a protection command to the switching equipment |
| Circuit Breaker | Disconnects the protected circuit when the trip mechanism operates |
| PLC or Supervisory Controller | Receives available monitoring information where connected |
| Analog Input Module | Receives the 4–20 mA signal where the analog option is fitted |
| Communication Network | Carries supported measurement or status information where configured |
The relay’s effectiveness depends on correct voltage connections, suitable threshold settings, and a functioning output circuit. The final protection response is determined by the complete installation, including any interlocks and supervisory control logic.
Industrial Applications
| Application | Typical Use |
|---|---|
| Generator Protection Panels | Monitoring generator terminal voltage and identifying abnormal voltage conditions |
| Transformer Feeders | Supervising voltage conditions associated with transformer-fed circuits |
| Industrial Switchgear | Detecting undervoltage and overvoltage conditions on monitored supplies |
| Three-Phase Distribution Systems | Monitoring phase-voltage abnormalities |
| Motor Supply Systems | Supporting voltage supervision where the relay configuration is suitable |
| Auxiliary Power Systems | Monitoring AC supplies used by industrial equipment |
| Electrical Control Cabinets | Providing voltage-related alarm and trip signals |
| Power-System Retrofit Projects | Replacing an existing voltage relay after checking its electrical and functional configuration |
| Supervisory Monitoring Systems | Supplying voltage information through supported analog or serial interfaces |
The MRU3-11D should be selected according to the voltage system, grounding arrangement, required protection functions, and output requirements. Applications requiring frequency protection, differential protection, or other specialized functions may need additional protection devices.
Installation and Maintenance
- Verify the complete model number: Confirm MRU3-11D and check the installed unit’s configuration before disconnecting the existing relay.
- Confirm the nominal voltage: Match the relay’s voltage-measurement configuration to the monitored electrical system.
- Check the auxiliary supply: Verify the permitted supply type and terminal connections for the installed version.
- Inspect the mounting arrangement: Confirm that the housing and panel or DIN-rail installation match the actual relay.
- Isolate the monitored circuit: Follow approved electrical safety and lockout procedures before handling voltage-measurement wiring.
- Check phase connections: Verify phase identification, terminal assignments, and wiring continuity against the electrical drawings.
- Review undervoltage settings: Set the undervoltage threshold and delay according to the protected equipment’s operating requirements.
- Review overvoltage settings: Confirm the overvoltage threshold and delay against the system’s normal voltage range and protection study.
- Verify phase-related protection: Check the configured phase-monitoring functions and their associated operating limits.
- Inspect output wiring: Confirm that alarm, trip, and digital outputs connect to their intended external circuits.
- Test the complete protection sequence: Use suitable test equipment to verify voltage pickup, timing, output operation, and the connected trip response.
- Record final settings: Document nominal voltage, protection thresholds, operating delays, output assignments, and test results for future maintenance.
Voltage protection testing should be performed by qualified personnel using approved procedures. Before returning the equipment to service, verify both the relay’s response and the downstream alarm or trip circuit. Do not assume that a displayed voltage value alone proves the complete protection path is operational.
Compatible System Components
The MRU3-11D works with the voltage measurement circuits and external control equipment required by the specific installation. Component selection depends on the monitored voltage, auxiliary supply, output configuration, and whether analog or serial monitoring is required.
| Component | Function |
|---|---|
| Voltage Measurement Circuit | Supplies phase-voltage signals to the relay |
| Compatible Auxiliary Power Supply | Provides the required operating power |
| Circuit Breaker | Disconnects the protected circuit following a trip command |
| Trip Coil Circuit | Converts the relay output into a switching action |
| Alarm Indication Circuit | Provides an operator warning when configured |
| PLC Digital Input | Receives compatible relay status or alarm signals |
| PLC Analog Input | Receives the 4–20 mA signal where the analog option is fitted |
| RS485 Network | Supports communication where the installed relay configuration provides the interface |
| Terminal Blocks | Organize voltage, power, and control wiring |
| Protection Test Equipment | Verifies voltage thresholds, delays, and output behavior |
Recommended Related Models
The following Woodward devices are related to electrical voltage supervision and protection. They are not confirmed direct replacements for the MRU3-11D; verify the required protection functions, nominal voltage, wiring, and interfaces before selecting an alternative.
| Model | Product Type | Relevance |
|---|---|---|
| Woodward MRU3-11D | Voltage Protection Relay | Same model for matching an existing installation |
| Woodward MRU3 Series | Digital Voltage Protection Relay Family | Related voltage-monitoring configurations |
| Woodward MRI3 Series | Digital Overcurrent and Earth-Fault Protection Relay Family | Related electrical protection equipment |
| Woodward MRI4 Series | Digital Feeder Protection Relay Family | Related feeder protection applications |
| Woodward MFR13 | Multifunctional Relay | Related electrical measurement and protection equipment |
| Woodward UMT1 | Measuring Transducer | Related voltage and current measurement equipment |
Key Advantages
- Monitors three-phase voltage conditions.
- Supports configurable undervoltage and overvoltage protection.
- Provides phase-related protection functions where configured.
- Offers external alarm and trip signaling.
- Supports analog or serial monitoring where the required options are installed.
- Can be integrated into generator and industrial distribution protection schemes.
Technical FAQs
Which voltage abnormalities can the MRU3-11D detect?
The MRU3-11D is used for undervoltage, overvoltage, and configured phase-related voltage protection. The exact functions and operating limits should be checked against the installed configuration and protection settings.
How should the operating delay be selected?
The delay should account for normal switching events, expected voltage fluctuations, and the response requirements of the protected equipment. Settings must also coordinate with other protective devices so that a sustained abnormal condition is cleared without unnecessary trips during brief disturbances.
What should be checked if the relay reports an undervoltage condition while the supply appears normal?
Verify the voltage at the relay’s measurement terminals, phase wiring, nominal-voltage configuration, and the selected undervoltage threshold. If the relay’s measured input is correct but the output response is unexpected, review the delay and assigned output logic.
Can the MRU3-11D send voltage measurements to a PLC?
The documented configuration supports a 4–20 mA analog voltage signal and RS485/Modbus RTU communication where the corresponding options are fitted. Confirm the installed interface and output configuration before connecting it to the PLC, and match the receiving input’s scaling to the relay’s configured signal.